Re: [pvusers] battery size and charge

["Scott Willing" <willing@mb.sympatico.ca>]

On 23 Nov 2002 at 17:40, Bill Neill wrote:

> I have a battery pack made of three strings (1, 2, 3) of two (A,B) 6
> Volt TROJAN T105 220 Ahr, cost $60 each battery.
> 
> LOOKS SOMETHING LIKE THIS:
> 
> B-1A     B-2A     B-3A
> B-1B     B-2B     B-3B
> 
> What is the total Ahr of this pack?

Crudely put, current adds in parallel, remains the same in series. 
Same applies to (current x time).

So each series string is 220Amp-Hours, times 3 strings = 660Amp-
Hours.  

> What should be the typical charge current for a total charge in one
> bell curve (sun day)? I estimate it at 20 Amps x 3 = 60 Amps.

Forgive me if I'm off-base, but it sounds as if you're assuming you 
have to hit 60A at some point in the day to fully charge your 
batteries, which is not the case.

An Ampere is a measure of instantaneous rate. It is equivalent to 
"one Coloumb of charge per second" - a flow rate similar to "gallons 
per minute". Filling a battery bank is like filling a water tank - 
it's (instaneous rate x time) that does the job, and within reason, 
hitting a certain instantaneous rate is neither a requirement nor is 
it particularly significant - unless you run out of time. :-)

So you don't *have* to hit 60A, if that's what you were thinking. 
What you have to do is put in as many Amp-Hours (current x time) as 
you remove, plus a bit extra to cover losses etc.

To return to the water tank analogy, Amp-Hours are like gallons. Your 
capacity is 660 gallons. If you've taken out 200 gallons, you might 
fill the tank back up over 10 hours at 20 gallons/hour, or in 3 hours 
and 20 minutes at 60 gallons/hour.

There *is* the issue of maximum rate (current) to consider. Although 
I'd refer to the manufacturer's guidelines to be sure, the usual rule 
of thumb is about C/10H i.e. the capacity in Amp-Hours divided by 
10Hours. In your case, that's 660AHr/10Hrs = 66A, so 60A is in the 
ballpark for *maximum* recommended charging current.

The very most you ever want to discharge this bank is half, or 
330AHrs. If you want your batts to live a lot longer, you'll try not 
to discharge more than 70 or 80% routinely, i.e. only draw about 150 
to 200AHrs before recharging.

So you have to determine whether / not you can (a) live, on average, 
on this energy budget and (b) whether / not your array can, on 
average, supply it.
 
> If your answers are as I expect, then since the 5 pvs are at 22 Amps
> in the single charge panel I now use, how do I add 5 or 10 more pvs so
> as to get to the 60 Amps in one sun day?

To eliminate the bell-curve complication for the sake of discussion, 
let's say you get about 6 hours of good, solid sun per day (the 
middle of the bell curve) and that during this period you manage to 
average 20A from the array. So on a sunny day you can collect 180AHr. 
If every day was like this, then you could draw 150AHrs/day or a 
little more. You'd be laughing with the array you have, and your 
batteries (properly watered etc.) will have a good long life.

But if you want to be able to use more power on a daily basis, or you 
have less input than this (i.e. you don't average 6 hours of full sun 
every day) then you want to add some more panels.

It's all a question of (supply x time) vs (demand x time). Hey, if 
you weren't using any power at all, just one of your panels would 
eventually charge that battery bank to full capacity and keep it 
happy indefinitely.

If you decide you need more panels to provide the supply/demand 
balance, your charge controller must be able to handle the additional 
current. So, if it's maxed out now, you either need to buy a charge 
controller that can handle the current of all the panels, or add a 
second one to handle the new panels alone. (Charge controllers can be 
paralleled.)

If your original charge controller isn't very sophisticated (e.g. 
doesn't use three-stage charging, or doesn't provide the option of 
adding a temperature probe) you'd be much better off investing in a 
new one that does a better job, and has the capacity for the total 
panel output. 

A three-stage charger, compared to a single set-point voltage 
regulator type, will generally make better use of the available power 
from the array, whether or not you add more panels.

> Must I split the pack?

No justification for that move that I can see. Indeed it is more 
likely to be a drawback. Once the batts have been subjected to 
different usage patterns for a while, it's not a great idea to 
recombine them in a single bank.

BTW, our little home/office uses a ~800AHr battery bank (4 Trojan HC-
16's) with an array of roughly the same peak current as yours. We're 
at 50 degrees North latitude, too, so winter days are pretty short.

So far (we've only been here a few years) with this arrangement we've 
had to resort to a gas generator to charge the batteries 2 - 3 days 
out of the entire year. This happens when we've had 5 or 6 heavily 
overcast short winter days, we're getting close to 60% capacity 
remaining, and there's no sun in the forecast. 

Frankly, under these circumstances I could have twice as many panels 
and it wouldn't help. (When there ain't *no* sun, twice nothin' is 
still nothin'.) This was a lesson that didn't really sink in until I 
lived here.

In the summer we have more power than we know what to do with - I 
even use a toaster and cappucino-maker every morning, which isn't 
something I ever expected to be running on a such a modest system. In 
winter, the woodstove takes over for breakfast preparation. :-)

BTW, do you have an Amp-Hour meter in your system? It's a lot easier 
to get a grip on your actual supply / demand situation if you have 
one. When I bought this place, the system already had a good voltage 
/ current monitor, but you really have to understand what's going on 
and have a bit of experience to interpret that. With just a little 
bit of charge current, for example, a bad battery can show you a nice 
high voltage reading.

With a Amp-Hour meter you get a much better concept of what's going 
on. When we first moved in here I thought we needed more panels, but 
I decided to get an Amp-Hour meter first, and kept daily records for 
a while. This helped me to realize I that the weak link in my system 
was batteries. Replacing the battery bank completely transformed the 
system, whereas (in my case at that time - I'm not trying to make a 
blanket statement) putting that money into panels would have been a 
huge dissappointment.

Sincerely hoping I haven't further confused the issue,
-=s




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